Johannes points out:
<!-- TH is for headers, TD for data, but for cells acting as both use TD -->
From URL:
http://www.w3.org/TR/html401/struct/tables.html#h-11.2.6
While I am a little chagrined that it took me eight or nine years to become aware of this particular detail, I am actually quite grateful for that little nugget! But I have to ask, how authoritative is it? Is a comment within sample code actually meant to be normative? I note that I am hardly the first person to have the audacity to ask this question, but I do not find that the point has been clarified. Thanks!
http://lists.w3.org/Archives/Public/w3c-wai-gl/2004AprJun/0329.html